Present day electronic private automatic branch exchanges (PABX's) are bulky, costly and consume considerable power. These shortcomings can be reduced dramatically by implementing the PABX in the latest VLSI technology. Recently, a single chip PABX which can provide voice and data service to 40 subscribers and that can be interconnected to form a larger PABX system has been developed. The single chip PABX is described by M. Cooperman et al in "Experimental Single Chip PABX," IEEE Journal of Solid State Circuits, Vol. SC-21, No. 2, April 1986, pp. 247-251 and in U.S. Pat. No. 4,630,284 issued Dec. 16, 1986. The single chip PABX system utilizes digital communication for both voice and data between a central switch, the local single chip PABX, telephones and data terminals. The connection between each telephone and the local PABX utilizes twisted pair wiring operating in the time division duplex mode.
Although VLSI circuits can perform highly complex functions with extremely small geometries, they are very limited in power dissipation. Conventional digital line driving techniques employ terminations at the receiving end to prevent reflections. The line driver transistor geometry must be large and the driver must deliver several hundred milliwatts to the termination load. In order to integrate the line driving and line receiving functions into the single chip PABX, transmission lines that are unterminated at the receiving end are utilized. As a result of the interaction between the transmitted and reflected waveforms, power is dissipated only during logic transitions. The power averages ten milliwatts per line driver for a data rate of 310 kilobits per second and a maximum line length of 400 feet.
The single chip PABX system utilizes a communication protocol in which the PABX chip looks for the first bit of a response during a specified time interval. Asynchronous receivers can be eliminated because the distance is short and the bits arrive with enough predictability to be recovered. In order for transmitted messages and received responses to remain synchronized, transmission line delay cannot exceed the duration of one bit. If the line delay exceeds one bit, the receiving end is not able to determine which bit of a response is being received at a given time. Accordingly, the line length for a given data rate is limited.
For a PABX system, 40 phones are typically located within 400 feet of the PABX chip. However, it is desirable in some circumstances to operate the single chip PABX system over longer distances. A typical application of the system is in a small building or a department in a larger building. It cannot always be insured that the maximum distance will be 400 feet. In addition, the PABX chip can be utilized in the public telephone network where the distance between 40 phones is likely to be well in excess of 400 feet. Operation over longer distances can be achieved by reducing the data rate. However, this slows the operating speed of the system.
It is a general object of the present invention to provide improved methods and apparatus for data communication.
It is another object of the present invention to provide methods and apparatus for synchronizing responses to compensate for different line delays in a data communication system.
It is yet another object of the present invention to provide a low power telecommunication system having the capability to operate over relatively long distances.
It is still another object of the present invention to provide a telecommunication system adapted for integration on one or a few integrated circuit chips.